Display device and method for driving the same

ABSTRACT

A display device displaying a color by mixing light reflected by a first reflection element  22  and light reflected by a second reflection element  26  by additive color mixture, in which the light of a first wavelength reflected by the first reflection element and light of a second wavelength reflected by the second reflection element have a mutually complementary color relationship. Thus, the display device, which can make good black and white display by a simple structure and can be driven by a simple method, can be realized.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Divisional of application Ser. No. 10/868,539, filed Jun. 14,2004 now U.S. Pat. No. 7,385,656, which is a Continuation ofInternational Application No. PCT/JP02/08554, with an internationalfiling date of Aug. 26, 2002, which designating the United States ofAmerica.

TECHNICAL FIELD

The present invention relates to a display device, more specifically areflective display device, and a method for driving the same.

BACKGROUND ART

Generally, CRTs, transmittive liquid crystal displays with backlightsare generally used in the display devices of computers and mobiledevices. The displays of this type are the so-called emissive displayswhich include internal emission means.

Based on recent studies, it is proposed to preferably use non-emissivereflective display devices in terms of work efficiency and fatigue inreading texts, etc., on display. The reflective display device, whichrequires no internal emission means and uses natural light, etc. fordisplay, is good for the eye and effective to decrease the electricpower consumption.

To realize further lower electric power consumption, display deviceshaving the memorization ability to retain displayed information evenwhen their source power is turned off are expected.

As such display devices are proposed display devices using chiralnematic liquid crystals and cholesteric liquid crystals. Chiral nematicliquid crystals are liquid crystals comprising nematic liquid crystalsand chiral catalysts added to the nematic liquid crystals. Chiralnematic liquid crystals and cholesteric liquid crystals have acharacteristic of reflecting selectively light of specific wavelengths.

A proposed display device using a chiral nematic liquid crystal will beexplained with reference to FIGS. 35A-B. FIGS. 35A-B are a schematicview of the proposed display device using a chiral nematic liquidcrystal.

As shown in FIGS. 35A-B, a photo-absorbing layer 114 is formed on asubstrate 110 of glass. An electrode 112 of ITO (Indium-Tin-Oxide) isformed on the photo-absorbing layer 114. A substrate 118 of glass isformed on the substrate 110 with the photo-absorbing layer 114 and theelectrode 112 formed on, opposed to the substrate 110. An electrode 120of ITO is formed on the side of the substrate 110, which is opposed tothe electrode 112. A liquid crystal layer 122 of chiral nematic liquidcrystal is provided between the substrate 110 with the photo-absorbinglayer 114 and the electrode 112 formed on and the substrate 118 with theelectrode 120 formed on. Thus, the display device using the chiralnematic liquid crystal is constituted.

A display device using a chiral nemtaic liquid crystal is disclosed in,e.g., Japanese published unexamined patent application No. Hei06-507505.

Then, the operation of the display device using the chiral nematicliquid crystal will be explained.

FIG. 35A shows the planer state. In the planer state, that of theincident light, whose wavelength corresponds to a helical pitch of theliquid crystal molecules is reflected. A wavelength λ for a maximum on areflection spectrum is expressed byλ=n·pwherein an average refractive index of the liquid crystal is representedby n, and a helical pitch of the liquid crystal is represented by p.Wavelength band width Δλ of reflected light is expressed byΔλ=Δn·pwherein an isotropy of refractive index of liquid crystal is representedby Δn.

FIG. 35B shows the focalconic state. In the focalconic state, theincident light passes through the liquid crystal layer 122 of the chiralnematic liquid crystal and is absorbed by the photo-absorbing layer 114formed on the substrate 110. Accordingly, in the focalconic state, blackcolor is displayed.

FIG. 36 is a graph of reflection spectra of the chiral nematic liquidcrystal. The wavelengths are taken on the horizontal axis, and on thevertical axis reflectances are taken. The reflectances on the verticalaxis were given when the reflection on a white reflection board is 100%.

The reflection wavelength of chiral nematic liquid crystals can be setat a prescribed value by suitably setting chiral catalyst amounts to beadded to the cholesteric liquid crystals. The addition of larger amountsof chiral catalysts decreases the helical pitches p of the liquidcrystals and shortens the wavelengths λ of the reflected light.

Chiral nematic liquid crystals have a characteristic of reflectingeither of right circularly polarized light and left circularly polarizedlight. This is described in, e.g., SID 97 DIGEST, p. 1019-1022.Characteristics of the chiral catalysts to be added to the chiralnematic liquid crystals can set the chiral nematic liquid crystals to beright circularly polarized light or left circularly polarized light.Chiral nematic liquid crystals, which reflect either of the rightcircularly polarized light and the left circularly polarized light,theoretically has the upper limit of the reflectance of 50%.

Planer and focalconic states are retained substantially permanentlyunless an external force is applied to the liquid crystals. Accordingly,the use of chiral nematic liquid crystals can provide display deviceshaving memorization ability which can retain displayed information evenwhen their power sources are turned off.

As described above, chiral nematic liquid crystals, which can constitutereflective display devices and can retain displayed information evenwhen the power sources are turned off, is noted as liquid crystals whichwill form the next generation display devices.

In a display device using a single layer of a chiral nematic liquidcrystal, in the planer state, light of a wavelength corresponding to thehelical pitch is selectively reflected, whereby the display colors arechromatic. On the other hand, in the focalconic state, the incidentlight is absorbed by the photo-absorbing layer, whereby the displaycolor is black. Accordingly, the display device using the single layerof the chiral nematic liquid crystal can display chromatic colors orblack color but cannot display white color.

Techniques of displaying white color by chiral nematic liquid crystalsare proposed as follows.

Japanese published unexamined patent application No. Hei 09-503873discloses the technique of mixing a plurality of kinds of chiral nematicliquid crystals, whereby all the visible spectra of 400-700 nm iscovered to thereby display while color.

Japanese published unexamined patent application No. 2001-066627discloses the technique of providing four liquid crystals of R (red), G(green), B (blue) and Y (yellow), whereby substantially all the visiblespectra are covered to thereby display white color.

Japanese published unexamined patent application No. 2001-109012discloses that chiral nematic liquid crystals of three colors, RGB areused to display white color.

Japanese published unexamined patent application No. Hei 11-231339discloses that a chiral nematic liquid crystal layer which reflectsselectively light of yellow color is formed on a photo-absorbing layerwhich absorbs light of blue color to display white color.

Techniques of display white color by using light scattering infocalconic state are also proposed.

However, the reflection wavelength band of chiral nematic liquidcrystals has a full width at half maximum of about 70-110 nm. Thedisplay device disclosed in Japanese published unexamined patentapplication No. Hei 09-503873 cannot cover all the visible spectra onlyby mixing chiral nematic liquid crystals of, e.g., two kinds andaccordingly cannot display white color. Furthermore, such displaydevice, in which two or more kinds of liquid crystals are mixed in onepolymer, has a risk that the liquid crystals might be mixed with oneanother.

The display devices disclosed in Japanese published unexamined patentapplication No. 2001-066627 and Japanese published unexamined patentapplication No. 2001-109012 both requires three or more liquid crystallayers, which is a blocking factor for the cost reduction. These displaydevices have high drive voltages, and their drive methods arecomplicated.

The display device disclosed in Japanese published unexamined patentapplication No. Hei 11-231339 has blue and white display colors.Accordingly, the visibility of the display devices is low unsuitable toread documents, such as texts, etc.

The technique of displaying white color by the scattering of light infocalconic state has the reflectance which is as low as about 20%, andbright white color display cannot be obtained. Accordingly, the displaydevice using such technique cannot have high contrast.

As described above, none of the proposed techniques have been able toprovide inexpensive display devices having good white and blackdisplays.

Chiral nematic liquid crystals have a characteristic that as theobservation angle is increased, the selective reflection wavelengthsshift to the side of shorter wavelengths. Accordingly, in displaydevices using simply chiral nematic liquid crystals, hues of the displaycolors change depending on observation direction changes. FIG. 37 is aconceptual view of the observation angle change. For example, a displaywhich is in red when observed at the front changes to green as theobservation angle θ is increased, and changes to blue as the observationangle θ is further increased. For example, a display which is in greenwhen observed at the front changes to blue as the observation angle θ isincreased. Monitor display devices are required to have a ±60°visibility range. The hue change in the ±60° range is not preferable.Accordingly, techniques of reducing the hue changes corresponding to theobservation angle changes have been expected.

DISCLOSURE OF INVENTION

An object of the present invention is to provide an inexpensivereflective display device which can make good black and white displaysand has little hue change corresponding to the observation directionchanges. Another object of the present invention is to provide a methodfor driving the display device, which is simple and can realize gooddisplays.

The above-described object is attained by a display device displaying acolor by mixing light reflected by a first reflection element and lightreflected by a second reflection element by additive color mixture, inwhich the light having a first wavelength reflected by the firstreflection element, and the light having a second wavelength reflectedby the second reflection element have substantially mutuallycomplementary color relationship.

The above-described object is also attained by a method for driving adisplay device including a first reflection element having a selectivereflection wavelength in a 480-500 nm range and a second reflectionelement having a selective reflection wavelength in a 580-640 nm range,and displaying a color by mixing light reflected by the first reflectionelement and light reflected by the second reflection element by additivecolor mixture, in which a display state of the first reflection elementand a display state of the second reflection element being both changedto switch between a white color display and a black color display.

The above-described object is also attained by a method for driving adisplay device including a first reflection element having a selectivereflection wavelength in a 450-480 nm range and a second reflectionelement having a selective reflection wavelength in a 570-610 nm range,and displaying a color by mixing light reflected by the first reflectionelement and light reflected by the second reflection element by additivecolor mixture, in which a display state of the first reflection elementbeing fixed, and a display state of the second reflection element beingchanged to switch between a white color display and a blue color displayor between a yellow color display and a black color display.

According to the present invention, reflection lights reflected on twoliquid crystal layers mutually have a complementary color relationship,whereby a display device which can realize good black and white displaycan be provided.

According to the present invention, the selective reflection wavelengthof one of the liquid crystal layers is in a 480-500 nm range, and theother of the liquid crystal layers in a 580-640 nm range, wherebychanges of the selective reflection wavelengths generated due toobservation angle changes can be compensated. A display device whichmakes no substantial change in hues of the display colors even when theobservation directions are changed can be provided.

According to the present invention, good black and white display can berealized by providing only two liquid crystal layers, whereby aninexpensive display device can be provided.

According to the present invention, the threshold voltages of therespective liquid crystal layers can be made substantially even, wherebya display device can have homogeneous display quality and much improvedstability in the drive.

According to the present invention, the selective reflection wavelengthof one of the liquid crystal layers is in a 450-480 nm range, and theselective reflection wavelength of the other of the liquid crystallayers is in a 570-610 nm range, whereby one alone of the liquid crystallayers is driven, whereby a display device which can make white-bluecolor display or yellow-black color display can be realized. Thus, thedisplay device can have a simple structure and can be driven by a simplemethod.

According to the present invention, chiral nematic liquid crystals areused, whereby displayed information can be retained even when theelectric power is turned off. Accordingly, the present invention canprovide a display device whose electric power consumption is low and hasmemorization ability.

According to the present invention, one of the liquid crystal layersuses the R liquid crystal, which reflects right circularly polarizedlight, and the other liquid crystal layer uses the L liquid crystal,which reflects right circularly polarized light. Accordingly, even whena wavelength bans in which a reflection spectrum of one liquid crystallayer and a reflection spectrum of the other liquid crystal layeroverlap each other present, light of a wavelength band which isreflected on one liquid crystal layer can be prevented from beingreflected on the other liquid crystal layer. Thus, according to thepresent invention, when a reflection spectrum of one liquid crystallayer and a reflection spectrum of the other liquid crystal layeroverlap each other, the decrease of the light reflection on either ofthe liquid crystal layers can be prevented, whereby the luminosity ofthe white color display can be increased.

According to the present invention, the liquid crystal layer of the Rliquid crystal and the liquid crystal layer of the L liquid crystal areformed for the respective selective reflection wavelengths λ₁, λ₂,whereby both the right circularly polarized light and the leftcircularly polarized light can be reflected. Accordingly, the presentinvention can provide a display device which can reflect the incidentlight with high efficiency and can provide white color display of higherluminosity.

According to the present invention, the substrate and the partitionlayer are formed of film, whereby a display device which is flexible andcan be used in extensive purpose.

According to the present invention, the thickness of the partitionlayer, etc. are so set that a phase difference between ordinary rays andextraordinary rays entering the liquid crystal layers is odd times λ₂/2,which permits good displays to be made even when films havingbirefringence are used.

According to the present invention, the thickness of the partition layeris so set that a phase difference between ordinary rays andextraordinary rays entering the liquid crystal layers is odd times λ₂/2,which permits good displays to be made even when the R liquid crystaland the L liquid crystal are combined.

According to the present invention, the liquid crystal layers aremicro-capsuled, whereby chiral nematic liquid crystals are preventedfrom mixing with each other even without the use of a partition layer.According to the present invention, it is not necessary to use apartition layer, which permits a display device to be thinned.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an xy chromaticity diagram explaining the principle of thepresent invention (Part 1).

FIG. 2 is an xy chromaticity diagram explaining the principle of thepresent invention (Part 2).

FIG. 3 is a sectional view of the display device according to a firstembodiment of the present invention, which shows a structure thereof.

FIG. 4 is a graph of reflection spectra of the white color display ofthe display device according to the first embodiment of the presentinvention.

FIG. 5 is a sectional view of the display device according to a secondembodiment of the present invention, which shows a structure thereof.

FIG. 6 is a graph of reflection spectra of the white color display ofthe display device according to the second embodiment of the presentinvention.

FIG. 7 is a sectional view of the display device according to onemodification of the second embodiment of the present invention, whichshows a structure thereof.

FIG. 8 is a sectional view of the display device according to a thirdembodiment of the present invention, which shows a structure thereof.

FIG. 9 is a sectional view of the display device according to onemodification of the third embodiment of the present invention, whichshows a structure thereof.

FIG. 10 is a sectional view of the display device according to a fourthembodiment of the present invention, which shows a structure thereof.

FIG. 11 is a sectional view of the display device according to a fifthembodiment of the present invention, which shows a structure thereof.

FIG. 12 is a circuit diagram of an equivalent circuit of a liquidcrystal cell.

FIG. 13 is a view of relationships between an applied pulse and avoltage applied to the liquid crystal layers.

FIGS. 14A-B are views of relationships between the resistivity of theliquid crystal and a voltage applied to the liquid crystal layers.

FIGS. 15A-B are views of state changes of the liquid crystal layersgiven when the threshold voltages of the liquid crystal layers aredifferent from each other.

FIGS. 16A-B are views of state changes of the liquid crystal layersgiven when the threshold voltages of the liquid crystal layers aresubstantially equal to each other.

FIGS. 17A-B are graphs of the response characteristics of the liquidcrystal layers given when the threshold voltage difference between theliquid crystal layers is large and small.

FIGS. 18A-B are graphs of the voltage characteristics of the respectiveliquid crystal layers containing no additive.

FIG. 19 is a circuit diagram used in measuring the partial voltageratios of the respective liquid crystal layers.

FIGS. 20A-B are graphs of the partial voltage ratios of the respectiveliquid crystal layers measured with ac pulses applied.

FIG. 21 is a graph of the measured voltage response of the displaydevice according to the fifth embodiment of the present invention.

FIG. 22 is a view explaining the structure and the operation of thephotoconductive layer.

FIGS. 23A-B are graphs showing the principle of the optical writingmethod using the photoconductive layer.

FIG. 24 is a sectional view of the display device according to a sixthembodiment of the present invention, which shows a structure thereof.

FIG. 25 is a sectional view of the display device according to a seventhembodiment of the present invention, which shows a structure thereof.

FIG. 26 is a graph of a spectral luminous efficacy curve of the eye ofman.

FIG. 27 is a graph of relationships between the width of the reflectionband of the blue color layer and the luminosity of blue color.

FIG. 28 is a graph of relationships between the width of the reflectionband of the blue color layer and the luminosity of white color.

FIG. 29 is a graph of relationships between the width of the reflectionband of the blue color layer and the contrast.

FIG. 30 is a graph of reflection spectra of the white color of thedisplay device according to the seventh embodiment of the presentinvention.

FIG. 31 is a graph of reflection spectra of the yellow color of thedisplay device according to the seventh embodiment of the presentinvention.

FIG. 32 is a sectional view of the display device according to an eighthembodiment of the present invention, which shows a structure thereof.

FIG. 33 is a sectional view of the display device according to a ninthembodiment of the present invention, which shows a structure thereof.

FIG. 34 is a view of one example of suitable ratio of partial voltagesapplied to the respective layers given when a voltage is applied betweenthe electrodes.

FIGS. 35A-B are diagrammatic views of the proposed display device usinga chiral nematic liquid crystal.

FIG. 36 is a graph of reflection spectra of the chiral nematic liquidcrystal.

FIG. 37 is a schematic view as watched in different observationdirections.

BEST MODES FOR CARRYING OUT THE INVENTION

[The Principle of the Present Invention]

The principle of the present invention will be explained with referenceto FIGS. 1 and 2. FIGS. 1 and 2 are the xy chromaticity diagrams showingthe principle of the present invention.

The inventors of the present invention have made earnest studies anddiscovered that two kinds of chiral nematic liquid crystals, one ofwhich has a 480-500 nm selective reflection wavelength and the other ofwhich has a 580-640 nm selective reflection wavelength, can be combinedso that colors of light corresponding to the selective reflectionwavelengths are complimentary colors to each other, whereby good blackand while displays can be realized, and the hue changes of the displaycolors due to observation direction changes can be prevented.

A color produced by mixing 2 kinds of color light by additive colormixture is a color corresponding to the central coordinates of a linesegment between two chromaticity coordinates in the xy chromaticitydiagram. When a selective reflection wavelength λ₁ of one chiral nematicliquid crystal is, e.g. 490 nm, and a selective reflection wavelength λ₂of the other chiral nematic liquid crystal is, e.g., 600 nm, the centralcoordinates of the line segment interconnection the two chromaticitycoordinates is coordinates corresponding to white color. Accordingly,two kinds of chiral nematic liquid crystals colors of the lightcorresponding to selective reflection wavelengths of which have thecomplementary color relationship with each other are combined, wherebygood white color can be displayed.

As shown in FIG. 2, as an observation angle is increased, selectivereflection wavelengths λ₁, λ₂ of the chiral nematic liquid crystalsrespectively shift to the shorter wavelength side. The selectivereflection wavelengths λ₁, λ₂ of the chiral nematic liquid crystals bothshift to the shorter wavelength side, and the central coordinates of theline segment interconnecting the two chromaticity coordinates in the xychromaticity diagram do not substantially change. Accordingly, even whenthe selective reflection wavelengths λ₁, λ₂ of the chiral nematic liquidcrystals respectively shift to the shorter wavelength side, theinfluences of the changes of the selective reflection wavelengths λ₁, λ₂on the display color are mutually compensated, and the hue of thedisplay color given by the additive color mixture make no substantialchange. Thus, chiral nematic liquid crystals having such selectivereflection wavelengths λ₁, λ₂ are combined, whereby a display devicehues of the display colors make no substantial change with changes ofthe observation directions can be provided.

In the above, the selective reflection wavelength λ₁ of one chiralnematic liquid crystal is 490 nm, and the selective reflectionwavelength λ₂ of the other chiral nematic liquid crystal is 600 nm, butthe combination of the selective reflection wavelengths of the chiralnematic liquid crystals is not essentially limited to the above. Theselective reflection wavelength λ₁ of one chiral nematic liquid crystalis within a range of 480-500 nm, and the selective reflection wavelengthλ₂ of the other chiral nematic liquid crystal is within a range of580-640 nm. Furthermore, these two kinds of chiral nematic liquidcrystals are combined so that colors of the light corresponding to theseselective reflection wavelengths λ₁, λ₂ have the complementary colorrelationship. The selective reflection wavelength λ₁ of one chiralnematic liquid crystal must be within a range of 480-500 nm, and theselective reflection wavelength λ₂ of the other chiral nematic liquidcrystal must be within a range of 580-640 nm, because unless theselective reflection wavelengths λ₁, λ₂ are within these ranges, thecentral coordinates of the line segment interconnection the twochromaticity coordinates cannot be coordinates corresponding to whitecolor, and the display color given by the additive color mixture cannotbe white color. Even if a display color near to white color is obtained,changes of the selective reflection wavelengths λ₁, λ₂ of the chiralnematic liquid crystals caused by observation direction changes cannotbe mutually compensated, and the hue of the display color changes withobservation direction changes.

As described above, according to the present invention, two kinds ofchiral nematic liquid crystals whose reflection light mutually have thecomplementary color relationship are combined, whereby a display devicewhich can display good white color can be provided. Furthermore,according to the present invention, the selective reflection wavelengthof one chiral nematic liquid crystal is within a range of 80-500 nm, andthe selective reflection wavelength of the other chiral nematic crystalis within a range of 580-640 nm, whereby changes of the selectivereflection wavelengths of the chiral nematic liquid crystals due toobservation direction changes can be compensated, whereby a displaydevice whose hues of display colors make no substantial change even withobservation direction changes can be provided. Furthermore, according tothe present invention, only two kinds of chiral nematic liquids are usedto display good black and white displays, whereby an inexpensive displaydevice can be provided.

A FIRST EMBODIMENT

The display device according to a first embodiment of the presentinvention will be explained with reference to FIG. 3. FIG. 3 is asectional view of the display device according to the presentembodiment, which shows the structure thereof.

As shown in FIG. 3, an electrode 12 of a 0.1 μm-thick ITO is formed on asubstrate 10 of glass. A photo-absorbing layer 14 of a 1 μm-thick isformed on the electrode 12. A 30 nm-thick partition layer 16 of glass isformed over the electrode 10, opposed to the substrate 10. A substrate18 of glass is formed over the partition layer 16, opposed to thepartition layer 16. An electrode 20 of ITO is formed on the side of thesubstrate 18, which is opposed to the partition layer 16.

For example, a 5 μm-thick liquid crystal layer 22 of a chiral nematicliquid crystal having a selective reflection wavelength λ₁ of 495 nm isformed between the substrate 18 with the electrode 20 formed on and thepartition layer 16. The liquid crystal layer 22 is sealed with a sealingcompound 24. The liquid crystal molecules of the chiral nematic liquidcrystal are twisted right. The chiral nematic liquid crystal(hereinafter temporarily called “R liquid crystal”) whose liquid crystalmolecules are twisted right reflects only right circularly polarizedlight.

The chiral nematic liquid crystal forming the liquid crystal layer 22can be formed by adding a chiral catalyst to a nematic liquid crystal.The nematic liquid crystal can be, e.g., E48 from Merck KGaA. The chiralcatalyst can be, e.g., CB15 from Merck KGaA. This chiral catalyst has acharacteristic of inducing liquid crystal molecules to twist right. Theselective reflection wavelength λ₁ of the chiral nematic liquid crystalcan be suitably set by adjusting the amount of the chiral catalyst to beadded to the nematic liquid crystal.

A liquid crystal layer 26 of a chiral nematic liquid crystal of, e.g., a5 μm-thick and a 601 nm selective reflection wavelength λ₂ is formedbetween the substrate 10 with the electrode 12 and the photo-absorbinglayer 14 are formed and the partition layer 16. The liquid crystal layer26 is sealed with a seal compound 28. The liquid crystal layer 26 isformed of the R liquid crystal. That is, the liquid crystal molecules ofthe chiral nematic liquid crystal forming the liquid crystal layer 26are twisted right.

The chiral nematic liquid crystal forming the liquid crystal layer 26can be formed by adding a chiral catalyst to a nematic liquid crystal,as described above. The nematic liquid crystal can be, e.g., E48 fromMerck KGaA, as described above. The chiral catalyst can be, e.g., CB15from Merck KGaA, as described above. The selective reflection wavelengthλ₂ of the chiral nematic liquid crystal can be suitably set by adjustingthe amount of the chiral catalyst to be added to the nematic liquidcrystal.

The liquid crystal display according to the present embodiment, whichincludes the liquid crystal layer 22 having the selective reflectionwavelength λ₁ on the side of the observation, and the liquid crystallayer 26 having the selective reflection wavelength λ₂ on the side ofthe photo-absorbing layer 14 is thus constituted.

Then, the operation of the display device according to the presentembodiment will be explained.

In the focalconic state, the incident light passes through the liquidcrystal layers 22, 26 and is absorbed by the photo-absorbing layer 14.Accordingly, in the focalconic state, the display color is black.

On the other hand, in the planer state, that of the incident light,which has wavelengths corresponding to helical pitches of the liquidcrystal molecules of the liquid crystal layers 22, 26 are selectivelyreflected on the liquid crystal layers 22, 26. The selective reflectionwavelength λ₁ of the liquid crystal layer 22 is 495 nm, and theselective reflection wavelength λ₂ of the liquid crystal layer 26 is 601nm. Accordingly, the display color given by mixing the reflection lightof the liquid crystal layers 22, 26 by the additive color mixture iswhite. Thus, in the planer state, the display color is white.

To change the chiral nematic liquid crystals of the liquid crystallayers 22, 26 from the focalconic state to the planer state, ac pulsesof, e.g., 500 V and 100 Hz are applied between the electrodes 12, 20.

To change the chiral nematic liquid crystals of the liquid crystallayers 22, 26 from the planar state to the focalconic state, ac pulsesof, e.g., 200 V and 100 Hz are applied between the electrodes 12, 20.

Generally, as the addition amount of the chiral catalyst is larger, thedrive voltage tends to be higher. The chiral catalyst is added in alarger amount to the liquid crystal layer 22, whose selective reflectionwavelength λ₁ is shorter than the liquid crystal layer 26, whoseselective reflection wavelength λ₂, in a larger amount. Accordingly, theliquid crystal layer 26, whose selective reflection wavelength λ₂ islong, is changed from the focalconic state to the planer state with alower applied voltage than in the case of the liquid crystal layer 22,whose selective reflection wavelength λ₁ is short. Accordingly, thechiral nematic liquid crystal alone of the liquid crystal layer 26 canbe changed from the focalconic state to the planer state by suitablysetting the voltage to be applied between the electrodes 12, 20. Thevoltage to be applied between the electrodes 12, 20 is suitably set,whereby a display device which can display not only black and while, butalso chromatic colors corresponding to the selective reflectionwavelength λ₂ can be provided.

(Evaluation Result)

Next, the results of the evaluation of the display device according tothe present embodiment will be explained.

(a) Reflection Spectra upon White Color Display

First, the reflection spectra upon the white color display will beexplained. FIG. 4 is a graph of the reflection spectra when the displaydevice according to the present embodiment displays white color. Thewavelengths are taken on the horizontal axis, and the reflectances aretaken on the vertical axis. A D65 light source was used in measuring thereflection spectra.

(b) Chromaticity upon White Color Display

Next, the chromaticity upon the white color display will be explained.

The chromaticity upon the white color display was measured, and theresult was x=0.319 and y=0.367.

Based on this, it is found that the present embodiment can display goodwhite color.

(c) Display Color Changes due to Observation Direction Changes

Next, the display color changes due to the observation direction changeswill be explained.

The display color changes due to the observation direction changes wereevaluated by changing the observation direction from 0° to 60° by 10° toobtain a maximum color difference Δu′v′ in a u′v′ uniform color space.

As Control 1, the maximum color difference Δu′v′ was measured on asingle liquid crystal layer which is arranged to display red color at anobservation angle of 0°. The result was that the maximum colordifference Δu′v′ was 0.162 in Control 1.

As Control 2, the maximum color difference Δu′v′ was measured on asingle liquid crystal layer which is arranged to display green color atan observation angle of 0°. The result was that the maximum colordifference Δu′v′ was 0.146 in Control 2.

As Control 3, the maximum color difference Δu′v′ was measured on asingle liquid crystal layer which is arranged to display blue color atan observation angle of 0°. The result was that the maximum colordifference Δu′v′ was 0.133 in Control 3.

As an example, the maximum color difference Δu′v′ was measured on thedisplay device according to the present embodiment. The result was thatthe maximum color difference Δu′v′ was 0.084.

Based on this, it can be seen that the display device according to thepresent embodiment can display colors which make no substantial changeeven with observation direction changes.

The present embodiment is thus arranged so that the reflection light onthe liquid crystal layer 22 and the reflection light on the liquidcrystal layer 26 mutually have the complementary color relationship,whereby the display device according to the present embodiment realizegood black and white display.

Furthermore, the display device according to the present embodiment, inwhich the selective reflection wavelength λ₁ of the liquid crystal layer22 is within a range of 480-500 nm, and the selective reflectionwavelength λ₂ of the liquid crystal layer 26 is within a range of580-640 nm, can compensate changes of the selective reflectionwavelengths λ₁, λ₂ due to the observation angle change and makes nosubstantial change in the hues of the display colors even withobservation direction changes.

According to the present embodiment, only two liquid crystal layers 22,26 are provides, whereby good black and white display can be make, andthe display device according to the present embodiment can beinexpensive.

According to the present embodiment, chiral nematic liquid crystals areused, whereby the display contents can be retained even when the powersource turned off. Accordingly, the display device according to thepresent embodiment can have low electric power consumption and havememorization ability.

A SECOND EMBODIMENT

The display device according to a second embodiment of the presentinvention will be explained with reference to FIG. 5. The same membersof the present embodiment as those of the display device according tothe first embodiment shown in FIG. 3 are represented by the samereference numbers not to repeat or to simplify their explanation.

FIG. 5 is a sectional view of the display device according to thepresent embodiment, which shows the structure thereof.

The display device according to the present embodiment is characterizedmainly in that one liquid crystal layer uses a chiral nematic liquidcrystal whose liquid crystal molecules are twisted right, and the otherliquid crystal layer uses a chiral nematic liquid crystal whose liquidcrystal molecules are twisted left (hereinafter temporarily called “Lliquid crystal”).

As in the first embodiment, a liquid crystal layer 22 of the R liquidcrystal is provided between a substrate 18 with an electrode 20 formedon and a partition layer 16.

On the other hand, a liquid crystal layer 26 a of the L liquid crystal,which is a chiral nematic liquid crystal whose liquid crystal moleculesare twisted left, is provided between a substrate 10 with an electrode12 and a photo-absorbing layer 14 formed on and the partition layer 16.The selective reflection wavelength λ₂ of the liquid crystal layer 26 ais set at, e.g., 601 nm. The thickness of the liquid crystal layer 26 ais, e.g., 5 μm, as is the thickness of the liquid crystal layer 26 ofthe display device according to the first embodiment shown in FIG. 3.The chiral nematic liquid crystal is formed by adding a chiral catalystto a nematic liquid crystal. The nematic liquid crystal can be, e.g.,E48 from Merck KGaA, as described above. The chiral catalyst can be,e.g., S811 from Merck KGaA. This chiral catalyst has a characteristicwhich induces liquid crystal molecules to twist left. The selectivereflection wavelength λ₂ of the chiral nematic liquid crystal can besuitably set by adjusting the amount of chiral catalyst to be added tothe nematic liquid crystal.

Thus, the display device according to the present embodiment isconstituted.

The display device according to the present embodiment is characterizedmainly in that, as described above, one liquid crystal layer 22 isformed of the R liquid crystal and the other liquid crystal layer 26 ais formed of the L liquid crystal.

In the display device according to the first embodiment, the incidentlight is reflected on the liquid crystal layer 22 in the wavelength bandwhere the reflection spectra of the liquid crystal layer 22 and thereflection spectra of the liquid crystal layer 26 overlap each other andis not reflected substantially on the liquid crystal layer 26.Accordingly, in the display device according to the first embodiment,less light is reflected on the liquid crystal layer 26.

In the present embodiment, however, the liquid crystal layer 22 isformed of the R liquid crystal, which reflects right circularlypolarized light, and the liquid crystal layer 26 a is formed of the Lliquid crystal, which reflect left circularly polarized light. In thepresent embodiment, even if the reflection spectra of the liquid crystallayer 22 and the reflection spectra of the liquid crystal layer 26 aoverlap each other is present, the light in the wavelength band wherethe light is reflected on the liquid crystal layer 26 a is preventedfrom being reflected on the liquid crystal layer 22. Thus, according tothe present embodiment, even when the reflection spectra of the liquidcrystal layer 22 and the reflection spectra of the liquid crystal layer26 overlap each other, the decrease of light to be reflected on theliquid crystal layer 26 a can be prevented, whereby the luminosity ofthe white display can be increased.

(Evaluation Result)

Next, the results of the evaluation of the display device according tothe present embodiment will be explained.

(a) Reflection Spectra upon the White Color Display

First, the reflection spectra upon the white color display will beexplained with reference to FIG. 6. FIG. 6 is a graph of the reflectionspectra upon the white color display. In measuring the reflectionspectra, a D65 light source was used as in the first embodiment.

As shown in FIG. 6, in the present embodiment, higher reflectances wereobtained in comparison with those of the reflection spectra of thedisplay device according to the first embodiment.

In comparing the luminosity of the white color display, the displaydevice according to the present embodiment had the luminosity of thewhite color display which was 1.4 times that of the display deviceaccording to the first embodiment.

Based on this, according to the present embodiment, the luminosity ofthe white color display can be higher.

(b) Chromaticity upon White Color Display

Then, the chromaticity upon the white color display will be explained.

The result of the chromaticity measured upon the white color display isx=0.328, y=0.350.

Based on this, according to the present embodiment, it is seen that goodwhite color display can be obtained.

(Modification)

The display device according to one modification of the presentembodiment will be explained with reference to FIG. 7. FIG. 7 is asectional view of the display device according to the presentmodification, which shows the structure thereof.

The display device according to the present modification ischaracterized mainly in that the device includes four liquid crystallayers.

As shown in FIG. 7, partition layers 30, 32 are formed, spaced from eachother between the substrate 18 with the electrode 20 formed on thepartition layer 16.

A liquid crystal layer 22 is formed between the substrate 18 with theelectrode 20 formed on and the partition layer 30. The liquid crystallayer 22 is formed of the R liquid crystal. The selective reflectionwavelength λ₁ of the liquid crystal layer 22 is set at, e.g., 492 nm.

The liquid crystal layer 22 a is formed between the partition layer 30and the partition layer 32. The liquid crystal layer 22 a is formed ofthe L liquid crystal. The selective reflection wavelength λ₁ of theliquid crystal layer 22 a is set at, e.g., 492 nm.

A liquid crystal layer 26 is formed between the partition layer 32 andthe partition layer 16. The liquid crystal layer 26 is formed of the Rliquid crystal. The selective reflection wavelength λ₂ of the liquidcrystal layer 26 is set at, e.g., 601 nm.

A liquid crystal layer 26 a is formed between the substrate 10 with theelectrode 12 and the photo-absorbing layer 14 formed on and thepartition layer 16. The liquid crystal layer 26 is formed of the Rliquid crystal. The selective reflection wavelength λ₂ the liquidcrystal layer 26 a is set at, e.g., 601 nm.

According to the present modification, liquid crystal layers 22, 26 ofthe R liquid crystal and the liquid crystal layers 22 a, 26 a of the Lliquid crystal are formed respectively for the selective reflectionwavelength λ₁ and the selective reflection wavelength λ₂, whereby boththe right circularly polarized light and the left circularly polarizedlight can be reflected. Thus, the display device according to thepresent embodiment can reflect the incident light with higher efficiencyand make brighter white color display.

A THIRD EMBODIMENT

The display device according to a third embodiment of the presentinvention will be explained with reference to FIG. 8. The same membersof the present embodiment as those of the display device according tothe first or the second embodiment shown in FIGS. 3 to 7 are representedby the same reference numbers not to repeat or to simplify theirexplanation.

FIG. 8 is a sectional view of the display device according to thepresent embodiment, which shows the structure thereof.

The display device according to the present embodiment is characterizedmainly in that the materials of the substrates and the partition layersare films.

As shown in FIG. 8, a substrate 10 a of film, a partition layer 16 a offilm, and a substrate 18 a of film are formed, opposed to each other.

A liquid crystal layer 22 is formed between the substrate 18 a with anelectrode 20 formed on and the partition layer 16 a. The liquid crystallayer 22 is formed of the R liquid crystal. The selective reflectionwavelength λ₁ of the liquid crystal layer 22 is set at, e.g., 492 nm.

A liquid crystal layer 26 is formed between the substrate 10 a with anelectrode 12 and a photo-absorbing layer 14 formed on and the partitionlayer 16 a. The liquid crystal layer 26 is formed of the R liquidcrystal. The selective reflection wavelength λ₂ of the liquid crystallayer 26 is set at, e.g., 601 nm.

In the display device according to the present embodiment, the materialsof the substrates 10 a, 18 a and the partition layer 16 a are films.Films generally have double refractivity. Accordingly, the simple use offilms as the materials of the substrates 10 a, 18 a and the partitionlayer 16 a cannot produce good display. In the present embodiment, thethickness, etc. of the partition layer 16 a is suitably set so that theordinary rays and the extraordinary rays entering the liquid crystallayer 26 have a phase difference which is odd times λ₂/2. The thickness,etc. of the partition layer 16 a are set to satisfy such conditions,whereby good display can be realized even in a case that films havingdouble refractivity are used.

The ordinary rays and the extraordinary rays entering the liquid crystallayer 26 are arranged to have a phase difference which is odd timesλ₂/2, whereby the left circularly polarized light passing through theliquid crystal layer 22 and entering the partition layer 16 a becomesright circularly polarized light when the former enters the liquidcrystal layer 26. Accordingly, the liquid crystal layer 22 reflects theright circularly polarized light, but the liquid crystal layer 26reflects the circularly polarized light which has been left wise whenentering the partition layer 16 a. Thus, according to the presentembodiment, even if the reflection spectra of the liquid crystal layer22 and the reflection spectra of the liquid crystal layer 26 overlapeach other, the decrease of light to be reflected on the liquid crystallayer 26 can be prevented, and bright white color display can beobtained.

As described above, according to the present embodiment, the substrates10 a, 18 a and the partition layer 16 a are formed of films, whereby thedisplay device can be used in flexibly wide applications.

According to the present embodiment, the thickness, etc. of thepartition layer 16 a are set so that the ordinary rays and extraordinaryrays entering the liquid crystal layer 26 have a phase difference whichis odd times λ₂/2, whereby the display device using even films havingdouble refractivity can realize good display.

(Modification)

Then, the display device according to one modification of the displaydevice according to the present embodiment will be explained withreference to FIG. 9.

The display device according to the present modification ischaracterized mainly in that the R liquid crystal and the L liquidcrystal are used in combination.

As in the display device according to the third embodiment shown in FIG.8, the liquid crystal layer 22 is formed between the substrate 18 a withthe electrode 20 formed on and the partition layer 16 a. The liquidcrystal layer 22 is formed of the R liquid crystal. The selectivereflection wavelength λ₁ of the liquid crystal layer 22 is, e.g., 492nm.

As in the display device according to the third embodiment shown in FIG.8, the liquid crystal layer 26 a is formed between the substrate 10 awith the electrode 12 and the photo-absorbing layer 14 formed on and thepartition layer 16 a. The liquid crystal layer 26 a is formed of the Lliquid crystal. The selective reflection wavelength λ₂ of the liquidcrystal layer 26 a is, e.g., 601 nm.

According to the present modification, the thickness, etc. of thepartition layer 16 a is suitably set so that the ordinary rays and theextraordinary rays entering the liquid crystal layer 26 a have a phasedifference which is even times λ₂/2. The thickness, etc. of thepartition layer 16 a are set to satisfy such condition, whereby gooddisplay can be obtained even if films having double refractivity areused.

The thickness, etc. of the partition layer 16 a are set so that a phasedifference between the ordinary rays and the extraordinary rays enteringthe liquid crystal layer 26 a is even times λ₂/2, whereby the leftcircularly polarized light passing through the liquid crystal layer 22and entering the partition layer 16 a remains left circularly polarizedwhen entering the liquid crystal layer 26 a. Accordingly, the liquidcrystal layer 22 can reflect the right circularly polarized light, andthe liquid crystal layer 26 a can reflect the left circularly polarizedlight.

Thus, according to the present modification, even if the reflectionspectra of the liquid crystal layer 22 and the reflection spectra of theliquid crystal layer 26 overlap each other, the decrease of light to bereflected on the liquid crystal layer 26 a can be prevented, and brightwhite display can be obtained.

As described above, according to the present modification, thethickness, etc. of the partition layer 16 a are set so that a phasedifference between the ordinary rays and the extraordinary rays enteringthe liquid crystal layer 26 a is even times λ₂/2, whereby even in thecombination of the R liquid crystal and the L liquid crystal, gooddisplay can be realized.

A FOURTH EMBODIMENT

The display device according to a fourth embodiment of the presentinvention will be explained with reference to FIG. 10. The same membersof the present embodiment as those of the display device according tothe first to the third embodiments shown in FIGS. 1 to 9 are representedby the same reference numbers not to repeat or to simplify theirexplanation.

FIG. 10 is a sectional view of the display device according to thepresent embodiment, which shows the structure thereof.

The display device according to the present embodiment is characterizedmainly in that a liquid crystal layer is in the form of microcapsules ofchiral nematic liquid crystals.

As shown in FIG. 10, a liquid crystal layer 22 in the form ofmicrocapsules and a liquid crystal layer 26 a in the form ofmicrocapsules are provided between a substrate 10 with an electrode 12and a photo-absorbing layer 14 formed on and a substrate 18 with anelectrode 20 formed on. The liquid crystal layer 22 is formed of the Rliquid crystal whose selective reflection wavelength λ₁ is, e.g., 492nm. The liquid crystal layer 26 a is formed of the L liquid crystalwhose selective reflection wavelength λ₂ is, e.g., 601 nm.

According to the present embodiment, the liquid crystal layers 22, 26 aare in the form of microcapsules, which prevents without a partitionlayer the chiral nematic liquid crystals from mixing with each other.According to the present embodiment, the partition layer 16 is notrequired, which allows the display device to be thinner.

Furthermore, according to the present embodiment, the liquid crystallayer 22 is formed of the R liquid crystal, and the liquid crystal layer26 a is formed of the L liquid crystal, whereby even if the reflectionspectra of the liquid crystal layer 22 and the reflection spectra of theliquid crystal layer 26 a overlap with each other, the decrease of lightto be reflected on the liquid crystal layer 22 or the liquid crystallayer 26 a can be prevented, and good white display can be obtained.

A FIFTH EMBODIMENT

The display device according to a fifth embodiment of the presentinvention will be explained with reference to FIGS. 11 to 23. The samemembers of the present embodiment as those of the display deviceaccording to the first to the fourth embodiments shown in FIGS. 3 to 10are represented by the same reference numbers not to repeat or tosimplify their explanation.

First, the display device according to the present embodiment will beexplained with reference to FIG. 11. FIG. 11 is a sectional view of thedisplay device according to the present embodiment, which shows thestructure thereof.

An electrode 12 is formed on a substrate 10. A photoconductive layer 34which generates charges by the application of light is formed. Aphoto-absorbing layer 14 is formed on the photoconductive layer 34. Overthe photo-absorbing layer 14, a partition layer 16 is formed,sandwiching a liquid crystal layer 26 a of the L liquid crystal. Overthe partition layer 16 an electrode 20 is formed, sandwiching a liquidcrystal layer 22 of the R liquid crystal. A substrate 18 is formed onthe electrode 20. The liquid crystal layers 26 a and the liquid crystallayer 22 are sealed respectively with seal compounds 28, 24.

The display device according to the present embodiment is characterizedmainly in that the liquid crystal layer 22 and the liquid crystal layer26 a are substantially equal to each other in the threshold voltage. Theeffect produced by making the threshold voltages of the liquid crystallayers 22, 26 a substantially equal to each other will be explained withreference to FIGS. 12 to 21.

FIG. 12 is a circuit diagram of an equivalent circuit of a liquidcrystal cell. FIG. 13 is a view of relationships between an appliedpulse and a voltage applied to the liquid crystal layers. FIGS. 14A-Bare views of relationships between the specific resistances of theliquid crystals and voltages applied to the liquid crystals. FIGS. 15A-Bare views of changes of the states of the liquid crystal layers givenwhen the threshold voltages of the liquid crystal layers are differentfrom each other. FIGS. 16A-B are views of changes of the states of theliquid crystal layers given when the threshold voltages of the liquidcrystal layers are substantially equal to each other. FIGS. 17A-B aregraphs of the response characteristics of the liquid crystal layersgiven when the threshold voltage difference between the liquid crystallayers is large and small. FIGS. 18A-B are graphs of the voltagecharacteristics of the liquid crystal layers containing no additive.FIG. 19 is a circuit diagram used in measuring the partial voltage ratioof the respective liquid crystal layers. FIGS. 20A-B are graphs of thepartial voltage ratios of the respective liquid crystal layers measuredwith alternate pulses are applied. FIG. 21 is a graph of the voltageresponse of the display device according to the present embodiment.

In the display device according to the first to the fourth embodiments,two or more liquid crystal layers are driven by a pair of electrodes.However, when the threshold voltages of the respective liquid crystallayers are different from each other, a contrast sufficiently utilizingthe potentials cannot be provided.

The threshold electric field strength E_(CN) of chiral nematic liquidcrystal is given byE _(CN)=(π² /P _(o))×(K22/∈₀Δ∈)^(1/2)wherein P_(o) represents a helical pitch; K₂₂, an elasticity constant oftwists; Δ∈, a dielectric constant anisotropy; and ∈₀, a vacuumdielectric constant. That is, there is a relationship that as thedielectric anisotropy is higher, the drive voltage is lower. Thedielectric anisotropy means that the dielectric constant of liquidcrystal molecules varies depending on a dielectric constant differencedue to directions of the axis of the liquid crystal molecules, i.e.,alignment states. In chiral nematic liquid crystals as well, theabove-described planer state, focal conic state and homeotropic statehave dielectric constants which are much different from each other. Inthe general liquid crystals, whose dielectric anisotropy is positive,the dielectric constant is maximum in the planer state, and minimum inthe homeotropic state.

Liquid crystals are not pure insulating films and have a property ofpassing a little current due to actions of ions, etc. generated inside.Accordingly, in an electric circuit equivalent to a liquid crystal cell,a liquid crystal cell can be substituted by the electric circuit havinga capacitor and a resistor connected in parallel as shown in FIG. 12.

Because of such property of liquid crystals, the voltage transition of aliquid crystal sandwiched between electrodes is as shown in FIG. 13. Theinitial value V₀ shown on the left in FIG. 13 corresponds to a value ofa dielectric constant of the capacitor. As the initial value V_(o) islarger, a higher voltage is required to charge the capacitor, and itmeans that the dielectric constant is low. The value of −Δv/Δt, V₀−Δvdepends on the specific resistance value of the liquid crystal.

That is, as shown in FIG. 14A, when the specific resistance of theliquid crystal is relatively high, the value of Δv is small, and theholding ability of the voltage is good. On the other hand, as shown inFIG. 14B, the holding ability of the voltage is degraded as the specificresistance value of the liquid crystal is smaller, which is a barrier tothe drive. Such specific resistance decrease is affected mainly by ions,etc. present in the liquid crystal.

Accordingly, when two liquid crystal layers, which are different in thedielectric constant anisotropy and the specific resistance, aresandwiched by a pair of electrodes, the following phenomena will takeplace when driven.

When a voltage is applied to the liquid crystal layers, more of thevoltage is divided to that of the liquid crystal layers, which has asmaller absolute value of the dielectric constant. For example, when a20 V voltage is applied to the layer structure of a liquid crystal layer22 having a dielectric constant of 8 in the planer state and a liquidcrystal layer 26 having a dielectric constant of 12 in the planer state,as shown in FIG. 15A, a 12 V voltage is applied to the liquid crystallayer 22, and an 8 V voltage is applied to the liquid crystal layer 26.

When the applied voltage is further increased, the liquid crystal layer22, to which more of the voltage is divided, reaches the thresholdvoltage and changes from the planer state to the homeotropic state. Theliquid crystal layer 22 has, e.g., a dielectric constant of 4 in thehomeotropic state. Accordingly, as shown in FIG. 15B, even when thevoltage to be applied to the layer structure is increased to, e.g., 32V, the voltage to be applied to the liquid crystal layer 26 is 8 V,which is lower than the threshold voltage, and only the voltage to beapplied to the liquid crystal layer 22 is increased to 24 V. Thus, whenthe dielectric constants of the liquid crystal layers are different fromeach other, it is very difficult to concurrently change states of boththe liquid crystal layers.

On the other hand, when the dielectric constants of the liquid crystallayers 22, 26 are equal to each other, as shown in FIGS. 16A and 16B,voltages to be applied to the liquid crystal layers 22, 26 aresubstantially equal to each other, and states of both the liquid crystallayers can be concurrently changed.

Accordingly, when a pair of electrodes are used, in order toconcurrently change states of a plurality of liquid crystal layers,i.e., make the threshold voltages substantially equal to each other, thedielectric constants of the respective liquid crystal layers are made asequal to each other as possible, and the specific resistances as wellare made as equal to each other as possible.

FIG. 17 is graphs of the response characteristics of the liquid crystallayers given when the threshold voltage difference between the liquidcrystal layers is large and small. As shown in FIG. 17A, when athreshold voltage difference is present between the liquid crystal layer1 and the liquid crystal layer 2, a movable range Vfc where both areturned into the focalconic state is small, but as shown in FIG. 17B,when the threshold voltages of the liquid crystal layer 1 and the liquidcrystal layer 2 are substantially equal to each other, the movable rangeVfc is wide, and the display quality and drive stability can be muchimproved.

Next, the method for controlling the threshold voltages of the liquidcrystal layers will be explained by means of examples.

As a liquid crystal which reflects the right circularly polarized light(the R liquid crystal), a suitable amount of a chiral catalyst CB15 forexciting the right helical structure is added to a liquid crystal E48from Merck KgaA to prepare a liquid crystal of a 492 nm dominantreflection wavelength. As a liquid crystal which reflects the leftcircularly polarized light (the L liquid), a suitable amount of a chiralcatalyst S811 for exciting the left helical structure was added to aliquid crystal E48 from Merck KGaA to prepare a liquid crystal of a 601nm dominant reflection wavelength.

FIG. 18 shows the voltage characteristics of the discrete liquidcrystals. FIG. 18A shows the response characteristics of the liquidcrystals changing from the planer state to the focalconic state. FIG.18B shows the response characteristics of the liquid crystals changingfrom the focalconic state to the planer state. As shown, the L liquidcrystal, which has a little lower amount ratio of the chiral catalyst,has a little lower drive voltage and oppositely the R liquid crystal,which has a higher amount ratio of the chiral catalyst, has a littlehigher drive voltage. A chiral nematic liquid crystal has a lowerspecific resistance and a higher drive voltage as the addition amount ofa chiral catalyst is larger.

Then, to electrically simulate the layer state of the liquid crystals,as shown in FIG. 19, glass cells are serially connected, ac pulses wereapplied, and the partial voltages of the respective liquid crystals wereinvestigated. The result is shown in FIG. 20. As shown in FIG. 20A, moreof the voltage is divided to the R liquid crystal, which has a largeramount ratio of the chiral catalyst and has a low absolute dielectricconstant, has a low apparent threshold voltage. Then, an about 3% of asurfactant was added to the L liquid crystal. The result is that, asshown in FIG. 20B, the addition of the surfactant could reverse therelationship of the partial voltage ratio. That is, the addition of asurfactant can much change properties of the liquid crystal layers, suchas dielectric constant and specific resistance.

Based on these results, a liquid crystal layer 26 a of the displaydevice according to the present embodiment shown in FIG. 11 comprisesthe L liquid crystal and 1.2% of a surfactant, TN-40 (from Asahi DenkaCo., Ltd.). The film thicknesses of the respective liquid crystal layers22, 26 a were 3 μm. The film thickness of a photoconductive layer 34 was10 μm.

The voltage response characteristic of the thus-constituted displaydevice according to the present embodiment was measured. The result isshown in FIG. 21. The threshold voltages of the liquid crystal layer 22and the liquid crystal layer 26 a are substantially in agreement witheach other, and good response characteristics could be realized.

Next, ac pulses of 500 V and 100 Hz were applied to this display device,and both liquid crystal layers 22, 26 a had the planer state. In theplaner state, the color mixture between the two layers produced goodwhite display.

Then, with a negative image mask applied to the surface on the side ofthe photoconductive layer 34, dc pulses of 80 V were applied generallyto the device while light is being applied. The two liquid crystallayers in regions to which the light has been applied to concurrentlychanged to the focalconic state and retained the planer state in regionsto which the light has not been applied. Vivid black and white displayof high contrast could be made.

Here, the mechanism for optical writing using the above-describedphotoconductive layer 34 will be explained with reference to FIGS. 22and 23A-B. FIG. 22 is a view explaining the structure and the operationof the photoconductive layer, and FIGS. 23A-B are graphs explaining themethod of the optical writing using the photoconductive layer.

To be specific, recently the photoconductive layer 34 generallycomprises, as shown in FIG. 22, a charge generating layer (CGL) 34 awhich generates charges by light application, and a charge transferlayer (CTL) 34 b which transfers the charges generated in the chargegenerating layer (CGL) 34 a.

When a photo-energy enters the CGL 34 a, precursors of charged carriers,having charge moments are generated and are divided into electrons andholes in the presence of electric fields. The CTL 34 b is usually formedof a hole-transfer type material, and the holes transfer in the CTL 34 bin the presence of electric field formed by electrified charges in thesurface of the photosensitizer. When an optical writing device combininga cholesteric liquid crystal and the photoconductive layer 34 is driven,the electrode on the side of the reflection layer is set −, and theelectrode on the side of the photoconductive layer 34 is set +.

The photoconductive layer 34 comprises an organic photosensitizer (OPC)or an inorganic material, such as amorphous silicon. The OPC is superiorto other photosensitizers in durability, processability and massproductivity, and is removably mounted on flexible media. The OPC hassuch a lot of merits and is a material which is presently most used.

Next, the display (reflectance) characteristic of the medium combining acholesteric reflection layer and the photoconductive layer will beexplained with reference to FIG. 23.

FIG. 23A is graph of the display characteristics in the drive from theplaner state to the focalconic state with light applied and withoutlight applied, which compare both cases with each other.

With light applied, when a voltage pulse signal exceeds a thresholdvoltage Vtf, the reflection layer goes on changing to the focalconicstate. When a voltage at which the reflection layer fully has thefocalconic state is Vfc, the focalconic state goes on changing again tothe planer state at voltage values exceeding the Vfc.

On the other hand, without no light applied, a threshold voltage Vtf′ atwhich the reflection layer starts to change to the focalconic state, anda voltage Vfc′ at which the reflection layer fully has the focalconicstate are largely rise in comparison with those with light applied.

Here, in comparison with the respective voltage values between withlight applied and without light applied, the voltage value Vfc at whichthe sufficient focalconic state is obtained with light applied is belowthe threshold voltage Vtf′ of the case without light applied. That is,the application of the voltage value Vfc changes the part the light hasbeen applied to the focalconic state, and the part the light has notbeen applied to retains the planer state.

FIG. 23B is a graph of the display characteristics in the drive from thefocalconic state to the planar state with light applied and withoutlight applied, which compare both cases with each other.

It is assumed that with light applied to, when an applied voltageexceeds Vtp, the liquid crystal goes on changing to the planer state andhas the complete planer state when the voltage is Vp.

On the other hand, it is assumed that with no light applied to, when anapplied voltage exceed Vtp′, the liquid crystal goes on changing to theplaner state and has the complete planer state when the voltage is Vp′.

In this case as well, in comparison of the respective voltage valuesbetween with light applied and without light applied, the thresholdvoltage much differs depending on whether or not light is being applied.For example, the general application of the voltage of Vp changes thepart where the light is applied to the planer state, but the partwithout the light application retains the focalconic state.

As described above, conductivity differences taking place in thephotoconductive layer after light application make different theelectric filed strength to be applied to the liquid crystal between thepart the light has been applied to and the part the light has not beenapplied to for the same applied voltage, whereby the liquid crystal canhave different states.

Such optical writing method is described in, e.g., the Japanesepublished unexamined patent application No. Hei 09-105900, SID 96Application Digest p. 59, “Reflective Display with Photoconductive Layerand Bistable, Reflective Cholesteric Mixture”, Japan Hardcopy 2000,“Electronic Paper using Cholesteric Liquid Crystal, Optical ImageWriting with Organic Photosensitizer”, etc.

As described above, according to the present embodiment, the thresholdvoltages of the respective liquid crystal layers are made substantiallyequal to each other, the display quality and drive stability of thedisplay device can be much improved.

In the present embodiment, the threshold voltages of the liquid crystallayers are controlled by the addition of a surfactant but may becontrolled by the addition of a material other than a surfactant. Forexample, the addition of a trace of an organic solvent (acetone, ethanolor others) has the effect of controlling the threshold voltages.However, a surfactant or a solvent should not be excessively contained,because there is a risk that the excessive content of a surfactant or asolvent will induce the crystallization (deposition) or denaturation ofthe liquid crystals.

In the present embodiment, the threshold voltages of the liquid crystalsare controlled by adding a surfactant to the liquid crystals, but thethreshold voltages can be controlled by other means.

For example, liquid crystals which are different in the dielectricconstant anisotropy may be blended in suitable amounts to thereby makethe threshold voltages equal to each other. The method of controllingthe threshold voltages by blending suitable amounts of liquid crystalswhich are different in the dielectric constant anisotropy will beexplained by means of a specific example.

As the R liquid crystal, a liquid crystal of dielectric constantanisotropy Δ∈ of 6.5 and a liquid crystal of anisotropy Δ∈ of 1.9 areblended with each other in a ratio of 1:2, and a chiral catalyst CB15for exciting the right helical structure is mixed in a suitable amount,and the dominant reflection wavelength was arranged to be 492 nm. As theL liquid crystal, a liquid crystal of dielectric anisotropy Δ∈ of 6.5and a liquid crystal of anisotropy Δ∈ of 1.9 are blended with each otherin a ratio of 3:2, and a chiral catalyst S811 for exciting the lefthelical structure is mixed in a suitable amount, and the dominantreflection wavelength was arranged to be 601 nm. The basic structure ofthe display device is as shown in FIG. 17, and the thicknesses of therespective liquid crystal layers were 3 μm.

The voltage response characteristics were measured on the thus prepareddisplay device. A good characteristic curve having the thresholdvoltages well agreed with each other was obtained.

Alternate current pulses of 500 V and 100 Hz were applied to thethus-prepared display device, and both liquid crystal layers 22, 26 ahad the planer state. In the planer state, the color mixture of the twolayers produced good white display.

Then, a negative image mask is applied to the surface on the side of thephotoconductive layer 34, and while light is being applied, dc pluses of80 V are applied generally to the device. The two liquid crystal layers22, 26 a in regions the light has been applied to changed to thefocalconic state and retained the planer state in regions the light hasnot been applied to. Vivid black and white display of high contrastcould be made.

A SIXTH EMBODIMENT

The display device according to a sixth embodiment of the presentinvention will be explained with reference to FIG. 24. The same membersof the present embodiment as those of the displayed device according tothe first to the third embodiments shown in FIG. 3 to 23 are representedby the same reference numbers not to repeat or to simplify theirexplanation.

FIG. 24 is a sectional view of the display device according to thepresent embodiment, which shows the structure thereof.

An electrode 12 is formed on a substrate 10. A photo-absorbing layer 14is formed on the electrode 12. An electrode 36 is formed over thephoto-absorbing layer 14, sandwiching a liquid crystal layer 26 a of theL liquid crystal therebetween. A substrate 38 is formed on the electrode36. An electrode 40 is formed on the substrate 38. An electrode 20 isformed over the electrode 40, sandwiching a liquid crystal layer 22 ofthe R liquid crystal therebetween. A substrate 18 is formed on theelectrode 20. The liquid crystal layer 26 a and the liquid crystal layer22 are sealed respectively with seal compounds 28, 24.

As described above, the display device according to the presentembodiment is characterized mainly in that the liquid crystal layers 22,26 a are sandwiched respectively by pairs of the electrodes, and theliquid crystal layer 22 and the liquid crystal layer 26 a can be drivenindependently of each other. The display device is thus structured,whereby the liquid crystal layers 22, 26 a can be controlledindependently in accordance with their respective properties, and thedisplay quality and drive stability can be drastically improved.

The threshold voltages of the liquid crystal layers 22, 26 a are notessentially equal to each other, but when in consideration of theperipheral circuits, controllability, etc., it is preferable that thethreshold voltages of the respective liquid crystals are substantiallyequal to each other, as in the display device according to the fifthembodiment.

According to the present embodiment, drive electrodes are formed foreach liquid crystal layer, whereby the display quality and drivestability can be drastically improved without considering the influenceof the partial voltages of the liquid crystal layers.

In the present embodiment, the display device according to the secondembodiment has a pair of electrodes for each liquid crystal layer, butthe display device according to the first or the third embodiment mayhave a pair of electrodes for each liquid crystal layer.

A SEVENTH EMBODIMENT

The display device according to a seventh embodiment of the presentinvention will be explained with reference to FIGS. 25 to 31. The samemembers of the present embodiment as those of the display deviceaccording to the first to the sixth embodiments shown in FIGS. 3 to 24are represented by the same reference numbers not to repeat or tosimplify their explanation.

In the first to the fourth embodiments, 2 layers which are mutuallycomplementary colors are laminated one on the other between a pair ofelectrodes to thereby display good white color when both layers have theplaner state and display black color when both layer have the focalconicstate. This method can provide vivid display of high contrast.

This method is structurally simpler than other various methods. However,this method requires 2 liquid crystal layers to be held between a pairof electrodes, which requires various contrivances as described in,e.g., the fifth and the sixth embodiments. Then, in the presentembodiment, a display device which is easier to drive than the displaydevice according to the first to the sixth embodiments will beexplained.

First, the liquid crystal display device according to the presentembodiment will be explained with reference to FIG. 25. FIG. 25 is asectional view of the display device according to the presentembodiment, which shows a structure thereof.

An electrode 12 is formed on a substrate 10. A photo-absorbing layer 14is formed on the electrode 12. An electrode 36 is formed over thephoto-absorbing layer 14 with a liquid crystal layer 26 a of the Lliquid crystal interposed therebetween. A substrate 38 is formed on theelectrode 36. An electrode 40 is formed on the substrate 38. Anelectrode 20 is formed over the electrode 40 with a liquid crystal layer22 of the R liquid crystal interposed therebetween. A substrate 18 isformed on the electrode 20. The liquid crystal layers 26 a, 22 aresealed respectively with seal compounds 28, 24.

The liquid crystal layer 22 is blue color layer having the dominantwavelength λ₁ of the reflection spectra of which is about 450-480 nm.The liquid crystal layer 26 a is yellow color layer having the dominantwavelength λ₂ of the reflection spectra of which is about 570-610 nm.The full width at half maximum of the reflection band of the blue colorlight on the liquid crystal layer 22 is 70 nm or less.

The display device according to the present embodiment includes suchliquid crystal layers 22, 26 a and drives the liquid crystal layer 26 awith the liquid crystal layer 22 fixed at the planer state or thefocalconic state, whereby, based on the additive color mixture,white-blue color display and yellow-black color display are realized.

When the liquid crystal layer 22 has the planer state, and the liquidcrystal layer 26 a has the planer state, white color display is made,and blue color display is made when the liquid crystal layer 26 a hasthe focalconic state. That is, the white-blue color display can berealized. On the other hand, when the liquid crystal layer 22 has thefocalconic state, and the liquid crystal layer 26 a has the planerstate, yellow display is made, and black color display is made when theliquid crystal layer 26 a has the focalconic state. That is, theyellow-black display can be realized.

As described above, the display device according to the presentembodiment can change over the display by driving the liquid crystallayer 26 a alone. The white-blue color display and the yellow-blackdisplay can be switched by only driving the liquid crystal layer 22.Accordingly, the control of the display device can be very simple. It isnot necessary to consider relationships between the liquid crystal layer22 and the liquid crystal layer 26 a, such as threshold voltages, etc.

Then, characteristics of the white-blue color display and theyellow-black color display will be explained with reference to FIGS. 26to 31. FIG. 26 is a spectral luminous efficacy curve of the eye of man.This graph shows that colors which have the same radiation energy butwhose wavelength is about 555 nm corresponding to yellowish green colorregion is most bright to the eye of man, and the visual sensitivitydecreases from the wavelength toward the shorter-wavelength side (bluecolor side) and to the longer-wavelength side (red color side).

Here, one of the 2 liquid crystal layers forming the display device isfixed at the planer state, and the other of the 2 liquid crystal layersis driven, whereby, the white-blue color display and the white-yellowcolor display can be made. In comparing the two displays, it can beintuitively understood that the white-blue color display, which displaysletters in blue, which is a cool color, can be read with lesspsychological stress than the white-yellow color display, which displaysletters in yellow, which is a warm color and has lower chroma.

This understanding based on the visibility can be explained based on thesmall area third color vision abnormality, which is characteristic ofman. The small area third color vision abnormality is the phenomena thatthe sensitivity to blue colors is low in small areas, and colors on theside of short wavelengths are invisible. That is, when small letters aredisplayed in blue colors, the sensitivity of the eye to the letters islower, whereby larger contrasts than actually measured can be sensed.

Based on the above, it can be understood that the white-blue colordisplay has more advantages than the white-yellow display and is next tothe white-black display in the visibility.

Then, it is theoretically explained that a narrower reflection band ofblue color can finally provide higher contrast.

A simulation of the white color display with an about 90 nm full widthat half maximum value of the yellow layer and a full width at halfmaximum value of the blue color layer as a parameter was made. FIG. 27shows the result of computation of the luminosity of the blue colorlayer for the reflection band (the full width at half maximum value) ofthe blue color layer as a parameter. As shown, as the reflection band ofthe blue color layer is decreased, the eyes of man feel darker. That is,it is shown that letters can be displayed thick.

The computation result shows that even with the full width at halfmaximum value of the yellow color layer changed, good white colordisplay can be obtained after laminated. That is, as shown in FIG. 28,the luminosity of the white color given by the color mixture was notdecreased and was substantially constant.

FIG. 29 is a graph of the contrast of the white-blue color display atthat time. As shown, as the full width at half maximum value of the bluecolor layer is smaller, the blue is darker, and the contrast is better.Oppositely, as the full width at half maximum value of the blue colorlayer is above about 70 nm, the contrast is less than 5, and thevisibility is lower.

Based on the above, it has been shown that the reflection band of theyellow color layer is wider, the reflection band of the blue color layeris narrower, whereby good white-blue color display can be obtained. Thatis, in the case that only one of the liquid crystal layers is driven toswitch the display, it is preferable to use the blue color layer as thefixed layer and the yellow color layer as the drive layer.

On the other hand, the blue color layer of the 2 layers forming thedisplay device is fixed at the focalconic state, and the yellow colorlayer is used in the drive, whereby the yellow-black color display ispossible. Yellow color, which can be sensed by the vision of man amonghues at low chroma because of the visional characteristic of man, gives,upon display, less stress due to the hue than other hues, such as red,green blue, etc., and the display having good visibility.

The evaluation result of the display device according to the presentembodiment will be explained. FIG. 30 is a graph of reflection spectraof white color in the display device according to the presentembodiment. FIG. 31 is a graph of reflection spectra of yellow color inthe display device according to the present embodiment.

The liquid crystal layer 22 was formed of the R liquid crystal which wasprepared by mixing a suitable amount of a chiral catalyst CB15, whichexcites the right helical structure, in a nematic liquid crystal ofΔn=0.25 and had an about 480 nm-dominant reflection wavelength. The fullwidth at half maximum value of the reflection band of the liquid crystallayer 22 was about 70 nm. The liquid crystal layer 26 a was formed ofthe L liquid crystal which was prepared by mixing a suitable amount of achiral catalyst S811, which excites the left helical structure, in anematic liquid crystal, and had a Δn=0.33 and had an about 590nm-dominant reflection wavelength. The full width at half maximum valueof the reflection band of the liquid crystal layer 26 a was about 105nm. The thicknesses of the liquid crystal layer 22 and the liquidcrystal layer 26 a were respectively 3 μm.

The display device shown in FIG. 25 was formed by using thethus-adjusted liquid crystals, ac pulses of 50 V, 100 Hz were appliedrespectively between the electrode 20 and the electrode 40 and betweenthe electrode 36 and the electrode 12 to place the liquid crystal layers22, 26 a into the planer state. In this state, the reflection spectrawere measured, and the reflection spectra shown in FIG. 30 were given.x=0.318, y=0.322 and Y=0.406 are obtained, and the color mixture of the2 layers gave good white color display.

Then, ac pulses of 20V, 100 Hz were applied between the electrode 36 andthe electrode 12 to place the liquid crystal layer 26 a in thefocalconic state. In this state reflection spectra were measured, andgood blue color display could be given.

The contrast ratio was about 18, and although the background color isnot white, vivid images of high contrast could be given.

Then, ac pulses of 20V, 100 Hz were applied between the electrode 20 andthe electrode 40 to place the liquid crystal layer 22 into thefocalconic state, and ac pulses of 50 V, 100 Hz were applied between theelectrode 36 and the electrode 12 to place the liquid crystal layer intothe planer state. In this state, the reflection spectra were measured,and the reflection spectra as shown FIG. 31 were given. x=0.518, y=0.424and Y=0.324 are obtained, and bright yellow display was given.

As described above, according to the present embodiment, a blue colorlayer having the dominant wavelength λ₁ of the reflection spectra ofwhich is about 450-480 nm is used as the first liquid crystal layer, theyellow color layer having the dominant wavelength λ₂ of the reflectionspectra of which is about 570-610 nm is used as the second liquidcrystal layer, the yellow color layer alone is used as the main drivelayer, whereby the display device which can make good white colordisplay by the simple structure and the simpler drive method can berealized.

The blue color layer is driven, whereby the white-blue color display andthe yellow-black color display can be easily switched. The full width athalf maximum value of the reflection band of blue light on the bluecolor layer is 70 nm or below, whereby the white-blue color display andthe black-yellow color display of good visibility can be made.

AN EIGHTH EMBODIMENT

The display device according to an eighth embodiment of the presentinvention will be explained with reference to FIG. 32. The same membersof the present embodiment as those of the display device according tothe first to the seventh embodiment shown in FIGS. 3 to 31 arerepresented by the same reference numbers not repeat or to simplifytheir explanation.

FIG. 32 is a sectional view of the display device according to thepresent embodiment, which shows the structure thereof.

The display device according to the present embodiment is the same inthe basic structure as the display device according to the seventhembodiment shown in FIG. 25. The display device according to the presentembodiment is characterized in that a photoconductive layer 34 isprovided between the electrode 12 and the liquid crystal layer 26 a asshown in FIG. 32. The presence of the photoconductive layer 34 permitsthe display device according to the seventh embodiment to opticallywrite images.

Next, the evaluation result of the display device according to thepresent embodiment will be explained.

The liquid crystal layer 22 was formed of the R liquid crystal having anabout 480 nm-dominant reflection wavelength prepared by mixing asuitable amount of a chiral catalyst CB15, which excites the righthelical structure, in a nematic liquid crystal of Δn=0.25. The fullwidth at half maximum value of the reflection band of the liquid crystallayer 22 was about 70 nm. The liquid crystal layer 26 a was formed ofthe L liquid crystal having an about 590 nm-dominant reflectionwavelength prepared by mixing a suitable amount of a chiral catalystS811, which excites the left helical structure, in a nematic liquidcrystal of Δn=0.33. The full width at half maximum value of thereflection band of the liquid crystal layer 26 a was about 105 nm. Thethicknesses of the liquid crystal layer 22 and the liquid crystal layer26 a were respectively 3 μm. The thickness of the photoconductive layer34 was generally 10 μm.

The display device shown in FIG. 32 was formed of the thus adjustedliquid crystals, and ac pulses of 50V, 100 Hz were applied between theelectrode 20 and the electrode 40 to place the liquid crystal layer 22into the planer state.

Next, with an image mask applied to the surface on the side of thephotoconductive layer 34, dc rectangular waves of 120 V were appliedbetween the electrode 36 and the electrode 12, whereby the liquidcrystal layer 26 a in the regions of the device where the light has beenapplied changed into the planer state, and the liquid crystal layer 26 ain the regions where the light has not been applied changed into thefocalconic state. Positive images of the white-blue color display ofgood visibility could be provided.

On the other hand, dc rectangular waves of 50 V were applied between theelectrode 36 and the electrode 12, whereby the liquid crystal layer 26 ain the regions of the device where the light has been applied changedinto the focalconic state, and the liquid crystal layer 26 a in theregion where the light has not been applied changed into the planerstate. Negative images of the white-blue color display of goodvisibility could be provided.

The contrast ratio was about 6.5, and the display of the level ofnewspapers, which can be seen without stress could be provided.

Then, ac pulses of 20 V, 100 Hz were applied between the electrode 20and the electrode 40 to place the liquid crystal layer 22 into thefocalconic state.

Next, with an image mask on the surface on the side of thephotoconductive layer 34, dc pulses of 100 V were between the electrode36 and the electrode 12, whereby the liquid crystal layer 26 a in theregions of the device where the light has not been applied changed intothe planer state, and the liquid crystal layer 26 a in the regions wherethe light has not been applied changed into the focalconic state.Positive images of the yellow-black color display of good visibilitycould be provided.

On the other hand, dc pulses of 50 V were applied between the electrode36 and the electrode 12, whereby the liquid crystal layer 26 a in theregions of the device where the light has not been applied changed intothe focalconic state, and the liquid crystal layer 26 a in the regionswhere the light has not been applied changed into the planer state.Negative images of the yellow-black color display of good visibilitycould be provided.

The contrast ratio was about 18, and although the background was notwhite, vivid images of high contrast could be provided.

As described above, according to the present embodiment, a blue colorlayer having the dominant wavelength λ₁ of the reflection spectra ofwhich is about 450-480 nm is used as the first liquid crystal layer, theyellow color layer having the dominant wavelength λ₂ of the reflectionspectra of which is about 570-610 nm is used as the second liquidcrystal layer, and the yellow color layer alone is used as the maindrive layer, whereby the display device which can make good white colordisplay by the simple structure and the simpler drive method can berealized. The optical writing using the photoconductive layer permitsthe white-blue color display and the black-yellow color display of goodvisibility to be made.

A NINTH EMBODIMENT

The display device according to a ninth embodiment of the presentinvention will be explained with reference to FIGS. 33 and 34. The samemembers of the present embodiment as those of the display deviceaccording to the first to the seventh embodiments shown in FIGS. 3 to 32are represented by the same reference numbers not to repeat or tosimplify their explanation.

FIG. 33 is a sectional view of the display device according to thepresent embodiment, which shows a structure thereof. FIG. 34 is a viewone example of a suitable ratio of partial voltages to be applied to therespective layers when a voltage is applied between the electrodes.

The display device according to the seventh and the eighth embodimentsincludes a pair of electrodes (the electrodes 20, 40) for driving theliquid crystal layer 22, and a pair of electrodes (the electrodes 12,36) for driving the liquid crystal layer 26 a. The display device can bearranged to make the white-blue color display and the yellow-black colordisplay by using one pair of drive electrodes. In the presentembodiment, such display device will be explained.

An electrode 12 is formed on a substrate 10. A photoconductive layer 34which generates charges by the application of light is formed on theelectrode 12. A photo-absorbing layer 14 is formed on thephotoconductive layer 34. A partition layer 16 is formed over thephoto-absorbing layer 14 with a liquid crystal layer 26 a of the Lliquid crystal interposed therebetween. An electrode 20 is formed overthe partition layer 16 with a liquid crystal layer 22 of the R liquidcrystal interposed therebetween. A substrate 18 is formed on theelectrode 20. The liquid crystal layer 26 a and the liquid crystal layer22 are sealed respectively with seal compounds 28, 24.

The display device according to the present embodiment is arranged todivide as follows a voltage applied to the respective layer.

FIG. 34 show one example of the suitable ratio of partial voltages of avoltage applied between the electrode 12 and the electrode 20 divided tothe respective layers. In this figure, the ratio of the thicknesses ofthe respective layers indicates the partial voltage ratio of thevoltage.

It is preferable that as a voltage Vo to be applied to thephotoconductive layer 34, many voltages are applied to increasedisplay/non-display contrast of letters and the S/N ratio (displaynoises). Oppositely, it is preferable that the partition layer 16 andthe photo-absorbing layer 14 are formed of materials of dielectricconstants as high as possible to thereby suppress the voltages Vd, Vkand the ratios of the partial voltages to the layers as low as possible.The voltage Vb to be applied to the liquid crystal layer 22 is madelower than the voltage Vy to be applied to the liquid crystal layer 26a. The resistivity of the liquid crystal layer 22 can be made lower byadding an additive, such as a surfactant or others, to the liquidcrystal layer 22 to thereby lower the ratio of the partial voltageapplied to the liquid crystal layer 22.

The partial voltage ratio of the partial voltages applied to therespective layers is thus controlled, whereby the liquid crystal layer26 a can be selectively driven by the voltage applied between theelectrode 12 and the electrode 30, and the display states can beswitched. The liquid crystal layer 22 can be also driven by applying ahigh voltage between the electrode 12 and the electrode 20, whereby thewhite-blue color display and the yellow-black color display can beswitched.

The liquid crystal layer 22, whose resistivity is lower, requires, forthe drive, a voltage which is far higher than a drive voltage requiredfor printing. However, the white-blue color display and the yellow-blackcolor display will not be frequently switched, and practically there isno problem.

The evaluation result of the display device according to the presentembodiment will be explained.

The liquid crystal layer 22 was formed of the R liquid crystal having anabout 480 nm-dominant reflection wavelength which was prepared by mixinga suitable amount of a chiral catalyst CB15, which excites the righthelical structure, in a nematic liquid crystal of Δn=0.25. A surfactantwas added by about 4% to this R liquid crystal to thereby much decreasethe resistivity.

The liquid crystal layer 26 a was formed of the L liquid crystal havingan about 590 nm-dominant reflection wavelength which was prepared bymixing a suitable amount of a chiral catalyst S811, which excites theleft helical structure, in a nematic liquid crystal of Δn=0.33.

The thicknesses of the liquid crystal layer 22 and the liquid crystallayer 26 a were respectively 3 μm. The thickness of the photoconductivelayer 34 was generally 12 μm.

The display device shown in FIG. 33 was formed of the thus-adjustedliquid crystals, and then with an image mask applied to the surface onthe side of the photoconductive layer 34, dc pulses of 500 V wereapplied between the electrode 12 and the electrode 20, whereby both theliquid crystal layer 22 and the liquid crystal layer 26 a wereinitialized into the planer state.

Then, with an image mask applied to the surface on the side of thephotoconductive layer 34, dc pulses of 150 V were applied between theelectrode 12 and the electrode 20, whereby the liquid crystal layer 26 aregions where the light has been applied changed into the focalconicstate, and positive images of the white-blue color display of goodvisibility could be provided.

With an image mask applied to the surface on the side of thephotoconductive layer 34, dc pulses of 300 V were applied between theelectrode 12 and the electrode 20, whereby the liquid crystal layer 22and the liquid crystal layer 26 a were initialized into the focalconicstate and the planer state, respectively.

Then, with an image mask applied to the surface on the side of thephotoconductive layer 34, dc pulses of 180 V were applied between theelectrode 12 and the electrode 20, whereby the liquid crystal layer 26 aregions where the light has been applied changed into the focalconicstate, and positive images of the yellow-black color display of goodvisibility could be provided.

As described above, in the display device according to the presentembodiment including a first liquid crystal layer is formed of a bluecolor layer having an bout 450-480 nm-dominant wavelength λ₁ of thereflection spectra and a second liquid crystal layer is formed of ayellow color layer having an about 570-610 nm-dominant wavelength λ₂ ofthe reflection spectra, and using the yellow color layer alone as thedrive layer, the partial voltage ratios of voltages to be applied to thefirst liquid crystal layer and the second liquid crystal layer are muchdiffered, which permits the display device to include a pair of driveelectrodes. The display device according to the present embodiment canhave further simpler structure than the display device according to theseventh and the eighth embodiments.

MODIFIED EMBODIMENTS

The present invention is not limited to the above-described embodimentsand can cover other various modifications.

For example, in the above-described embodiments, the liquid crystallayer of a selective reflection wavelength λ₁ is disposed on the side ofthe observation, the liquid crystal layer of a selective reflectionwavelength λ₂ is disposed on the side of the photo-absorbing layer 14,but it is possible that the liquid crystal layer of a selectivereflection wavelength λ₂ is disposed on the side of the observation, andthe liquid crystal layer of a selective reflection wavelength λ₁ isdisposed on the side of the photo-absorbing layer 14.

In the above-described embodiments, the chiral namatic liquid crystalsof the liquid crystal layers were changed from the focalconic state tothe planer state by applying a voltage between the electrodes 12, 20.However, the voltage application is not essential to change the chiralnematic liquid crystals from the focalconic state to the planer state.For example, the application of heat, mechanical stresses or others canchange the chiral nematic liquid crystals from the focalconic state tothe planer state.

In the first and the third embodiments, all the liquid crystal layersare formed of the R liquid crystal but may be formed of the L liquidcrystal.

In the second embodiment, the modification of the third embodiment, andthe fifth to the ninth embodiments, the liquid crystal layer of the Rliquid crystal of a selective reflection wavelength λ₁ is disposed onthe side of the observation, and the liquid crystal layer of the Lliquid crystal of a selective reflection wavelength λ₂ is disposed onthe side of the photo-absorbing layer 14. However, it is possible that aliquid crystal layer of the L liquid crystal of a selective wavelengthλ₁ is disposed on the side of the observation, and a liquid crystallayer of the R liquid crystal of a selective reflection wavelength λ₂ isdisposed on the side of the photo-absorbing layer 14.

In the above-described fourth embodiment, all the liquid crystal layersare micro-capsuled, but at least one of the liquid crystal layers may bemicro-capsuled. Micro-capsuling at least one of the liquid crystallayers can prevent a plurality of the liquid crystal layers from mixingwith each other.

The above-described fourth embodiment includes the liquid crystal layerof the R liquid crystal of a selective reflection wavelength λ₁ and theliquid crystal layer of the L liquid crystal of a selective reflectionwavelength λ₂ but may include a liquid crystal layer of the R liquidcrystal of a selective reflection wavelength λ₁, a liquid crystal layerof the L liquid crystal of a selective reflection wavelength λ₁, aliquid crystal layer of the R liquid crystal of a selective reflectionwavelength λ₂ and a liquid crystal layer of the L liquid crystal of aselective reflection wavelength λ₂. Thus, bright white color display canbe provided.

In the fifth, the eighth and the ninth embodiments, writing is performedby using the photoconductive layer. However, as in the display deviceaccording to the first to the fourth embodiments, image display madeperformed by using the voltage alone applied between the electrode 12and the electrode 20.

In the fifth, the eighth and the ninth embodiments, the photoconductivelayer is used to form regions of one liquid crystal layer, which havedifferent states. However, in the first to the fourth embodiments, thesixth embodiment and the seventh embodiment as well, regions of oneliquid crystal layer having different states can be formed, and in thiscase, at least one of a pair of electrodes for driving the liquidcrystals is formed in, e.g., a matrix so as to apply drive voltages torequired regions. Otherwise, the display device according to the firstto the third embodiments, and the sixth and the seventh embodiments mayinclude a photoconductive layer to display images by the method ofoptical writing.

In the fifth to the ninth embodiments, the substrates are exemplified byplate-like substrates, but films may be used as in the third embodiment.

The above-described embodiments have been explained by means of examplesusing chiral nematic liquid crystals, but chiral nematic liquid crystalsare not essential. Liquid crystals which are able to selectively reflectincident light can be used. For example, liquid crystals, such ascholesteric liquid crystal, etc., which can have the cholesteric phase,can be used.

In the above-described embodiments, liquid crystals are used, but liquidcrystals are not essentially used. For example, electrophoreticparticles may be used, or twist balls may be used. Electrophoreticparticles and twist balls are described in, e.g., Nikkei Microdevices,February, 2001.

INDUSTRIAL APPLICABILITY

In the display device according to the present invention in which lightreflected by first reflection means, and light reflected by a secondreflection means are mixed by additive color mixture, and the colors aredisplayed, light of a first wavelength to be reflected by the firstreflection means, and light of a second wavelength to be reflected bythe second reflection means mutually have a complementary colorrelationship, whereby good white and black display can be realized by asimple structure and a simple drive method. Accordingly, the displaydevice can usefully have low electric power consumption, andmemorization ability.

1. A method for driving a display device displaying a color by mixinglight reflected by a first liquid crystal region and light reflected bya second liquid crystal region by additive color mixture, wherein adominant wavelength of the light reflected by the first liquid crystalregion is in one of a range of 450-500 nm and a range of 570-640 nm, adominant wavelength of the light reflected by the second liquid crystalregion is in the other of the range of 450-500 nm and the range of570-640 nm, the light reflected by the first liquid crystal region, andthe light reflected by the second liquid crystal region have asubstantially mutually complementary color relationship, and areflection band of the first liquid crystal region and the second liquidcrystal region, which reflects light on a side of shorter wavelengthsbeing narrower than a reflection band of the first liquid crystal regionand the second liquid crystal region which reflects light on a side oflonger wavelengths, in which a display state of the first reflectionelement and a display state of the second reflection element being bothchanged to switch between a white color display and a black colordisplay.
 2. A method for driving a display device displaying a color bymixing light reflected by a first liquid crystal region and lightreflected by a second liquid crystal region by additive color mixture,wherein a dominant wavelength of the light reflected by the first liquidcrystal region is in one of a range of 450-500 nm and a range of 570-640nm, a dominant wavelength of the light reflected by the second liquidcrystal region is in the other of the range of 450-500 nm and the rangeof 570-640 nm, the light reflected by the first liquid crystal region,and the light reflected by the second liquid crystal region have asubstantially mutually complementary color relationship, and areflection band of the first liquid crystal region and the second liquidcrystal region, which reflects light on a side of shorter wavelengthsbeing narrower than a reflection band of the first liquid crystal regionand the second liquid crystal region which reflects light on a side oflonger wavelengths, in which a display state of the first reflectionelement being fixed, and a display state of the second reflectionelement being changed to switch between a white color display and a bluecolor display or between a yellow color display and a black colordisplay.
 3. A method for driving a display device according to claim 2,wherein the display state of the first reflection element is changed toswitch between a white-blue color display and a yellow-black colordisplay.